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Abstract:

A method, and related device, for operating a brake device of a vehicle
with brake slip regulation (ABS) on roadways with different friction
coefficients on different sides, as a result of which a braking yaw
moment is imparted to the vehicle during braking, characterized in that,
on at least one axle of the vehicle, an absolute brake pressure
difference between the, brake pressure at the wheel with the higher
friction coefficient and the brake pressure at the wheel with the lower
friction coefficient is adapted as a function of a steer input, intended
to produce a yaw moment acting counter to the braking yaw moment, by the
driver and/or by an automatically intervening auxiliary steering system.

Claims:

1-9. (canceled)

10. A method for operating a brake device of a vehicle with brake slip
regulation (ABS) on roadways with different friction coefficients on
different sides, as a result of which a braking yaw moment is imparted to
the vehicle during braking, the method comprising: adapting, on at least
one axle of the vehicle, an absolute brake pressure difference between
the brake pressure at the wheel with the higher friction coefficient and
the brake pressure at the wheel with the lower friction coefficient as a
function of a steer input, to produce a yaw moment acting counter to the
braking yaw moment, which is caused by at least one of the driver and an
automatically intervening auxiliary steering system.

11. The method of claim 11, wherein the brake pressure difference
permitted is larger, the larger the counter yaw moment produced by the
steer input, and in that the brake pressure difference permitted is
smaller, the smaller the counter yaw moment produced by the steer input.

12. The method of claim 11, wherein the adaptation of the brake pressure
difference is performed exclusively as a function of the steer input by
the driver.

13. The method of claim 11, wherein the adaptation of the brake pressure
difference is performed continuously as a function of the steer input.

14. The method of claim 11, wherein the adaptation of the brake pressure
difference is performed on a front axle brake device.

15. The method of claim 11, wherein the actual yaw rate of the vehicle is
adjusted to a setpoint yaw rate as a function of the steer input by
modifying the brake pressure difference.

16. The method of claim 15, wherein the setpoint yaw rate is
substantially equal to zero.

17. A device for operating a brake device of a vehicle with brake slip
regulation (ABS) on roadways with different friction coefficients on
different sides, as a result of which a braking yaw moment is imparted to
the vehicle during braking, comprising: at least one brake actuator per
wheel of the axle, the brake actuator being controllable electrically,
directly or indirectly, and being actuated by pressure medium; at least
one brake pressure sensor per brake actuator for producing sensor signals
dependent on the brake pressure acting in the respective brake actuator;
at least one steering angle sensor for producing sensor signals dependent
on the steer input; at least one yaw rate sensor for producing sensor
signals dependent on the yaw rate of the vehicle or on yaw rates acting
on the vehicle; and an electronic control unit configured to adapt the
brake pressure difference between the brake pressure at the wheel with
the higher friction coefficient and the brake pressure at the wheel with
the lower friction coefficient as a function of the sensor signals
produced by the steering angle sensor, the yaw rate sensor and the brake
pressure sensors.

18. The device of claim 17, further comprising: an auxiliary steering
system controlled by the control unit so that a yaw moment acting counter
to the braking yaw moment is produced at the steering wheel, adjusting
the difference between the actual yaw rate and the setpoint yaw rate to
zero.

Description:

FIELD OF THE INVENTION

[0001] A method for operating a brake device of a vehicle with an antilock
control system on roadways with different friction coefficients on
different sides, as a result of which a braking yaw moment is imparted to
the vehicle during braking.

BACKGROUND INFORMATION

[0002] In order to avoid a situation where the wheels of the motor vehicle
lock up when the brake is actuated due to an excessive brake pressure
applied by the vehicle driver and, as a result, the motor vehicle loses
its stability or steerability, vehicle brake systems are usually fitted
with a brake slip regulation system (ABS), in which the brake slip is
adjusted to an optimum brake slip.

[0003] In the case of brake slip regulation, the brake pressure is in each
case automatically modulated, i.e. lowered, held constant and raised
again, independently of the brake pedal force applied by the vehicle
driver, at least in part of the pressure-medium-actuated brake system, if
a risk of one or more vehicle wheels locking up is detected, until there
is no longer a risk of locking up. In very general terms, therefore,
brake slip regulating devices for motor vehicle brake systems have the
task of ensuring directional stability and steerability of the vehicle
combined with the shortest possible stopping and braking distances,
especially when the roadway is slippery and the service brake system is
fully applied.

[0004] On roadways with friction coefficients that are significantly
different as between the right and the left (split coefficient), however,
there may be a reduction in the directional or driving stability of the
vehicle owing to the very large difference in the effective braking
forces which then occurs at the right-hand and left-hand vehicle wheels.
This severe asymmetry or lack of balance in the effective braking forces
on the right-hand and left-hand side of the vehicle produces a braking
yaw moment of greater or lesser magnitude in accordance with these
asymmetrical forces, rotating the vehicle about the vertical axis
thereof. In order to counteract this and to maintain directional or
driving stability, i.e. to keep the vehicle on course, the vehicle driver
can actuate the steering wheel by way of correction in order to produce a
yaw moment acting counter to the braking yaw moment.

[0005] In the case of vehicle brake systems with protection against
locking up, there is therefore generally a conflict of aims in such
situations. On the one hand, the aim is to achieve braking and stopping
distances which are as short as possible when braking but, on the other
hand, it is also important to maintain directional or driving stability
and steerability of the vehicle when braking.

[0006] In this context, manufacturers of brake-slip-regulated motor
vehicle brake systems generally give higher priority to maintaining
directional or driving stability and steerability of the vehicle than to
achieving the shortest possible braking distances.

[0007] In order to maintain the directional and driving stability of the
vehicle, the ABS control strategy is adapted in such driving situations.
In this case, at least the wheels on one axle are subject to antilock
control on the "select-low" principle, for example, i.e. they are
controlled in dependence on the vehicle wheel currently operating with
the lowest friction coefficient. This means that, in the operating
situation described above, the brake of the wheel rotating on the higher
friction coefficient is supplied only with the same, comparatively low
brake pressure as the brake of the other wheel, that rotating on the
lower friction coefficient, even though it could in fact be braked more
strongly without locking up because of the higher friction coefficient
prevailing at this wheel. In this case, therefore, braking forces of the
same high magnitude or the same low magnitude are applied to both wheels,
the result being that they do not contribute anything to the production
of a braking yaw moment. Since the wheel rotating on the higher friction
coefficient is braked less strongly than is possible, it has a
correspondingly high potential to carry lateral forces, and this benefits
the directional or driving stability of the vehicle. However, the good
directional and driving stability is obtained at the expense of longer
braking distances since, with this control principle, the vehicle wheels
rotating on higher friction coefficients are braked less strongly than
would be permitted per se by the adhesion prevailing there.

[0008] Inasmuch as the two front wheel brakes in a hydraulic vehicle
antilock brake system with rear wheels protected from locking up by the
select-low principle are protected individually from locking up by
dedicated devices, it is customary to attenuate the effect of any yaw
moment that builds up due to braking forces of different magnitude at the
right-hand and left-hand front wheel by "yaw moment modification"
superimposed on the individual antilock control for the two front wheels.
The superimposed yaw moment modification ensures that the brake pressure
at the front wheel rotating on the higher friction coefficient is built
up more slowly than is possible per se in order to give the vehicle
driver additional time to respond, i.e. to countersteer, by the resulting
delayed buildup in the yaw moment. The superimposed yaw moment
modification also contributes to a deterioration in the achievable
braking or stopping distance.

[0009] According to DE 602 17 834 T2, there is an electrically assisted
steering system which intervenes during a split-coefficient braking
operation under brake slip regulation in order to keep the vehicle stable
by automatic steer inputs. By these stabilization measures, the ABS
behavior can be made more aggressive, with the brake pressure at the
wheel with the higher friction coefficient being increased more quickly,
i.e. at a higher rate, until the slip threshold is reached.

DISCLOSURE OF THE INVENTION

[0010] It is accordingly an object of the exemplary embodiments and/or
exemplary methods of the present invention to develop a method of the
type mentioned at the outset in such a way that, during a
split-coefficient braking operation under brake slip regulation, the
braking effect is as high as possible, on the one hand, but that, on the
other hand, driving stability is also as high as possible.

[0011] According to the exemplary embodiments and/or exemplary methods of
the present invention, this object is achieved by the features described
herein.

[0012] The exemplary embodiments and/or exemplary methods of the present
invention is intended to provide that, on at least one axle of the
vehicle, an absolute brake pressure difference between the brake pressure
at the wheel with the higher friction coefficient and the brake pressure
at the wheel with the lower friction coefficient is adapted as a function
of a steer input, intended to produce a yaw moment acting counter to the
braking yaw moment, by the driver and/or by an automatically intervening
auxiliary steering system.

[0013] In an exemplary embodiment, the brake pressure difference permitted
is larger, the larger the counter yaw moment produced by the steer input
or the larger the actual yaw rate resulting therefrom, and the brake
pressure difference permitted is smaller, the smaller the counter yaw
moment produced by the steer input or the smaller the actual yaw rate
resulting therefrom.

[0014] In other words, the permissible brake pressure difference at the
axle may be adapted continuously or in an infinitely variable manner to
the steer input by the driver and/or by the auxiliary steering system as
a response to the braking yaw moment produced during the
split-coefficient braking operation. Thus, the more the driver or the
auxiliary steering system counteracts the braking yaw moment by a steer
input, the larger is the brake pressure difference that can be permitted
between the wheels on different sides by the ABS controller. In this
case, a larger absolute brake pressure difference then means, for
example, that the wheel on the side with the higher friction coefficient
is braked with a higher braking force, leading ultimately to a greater
overall braking effect.

[0015] In contrast, e.g., according to DE 602 17 834 T2, only the brake
pressure gradient is modified as the stability increases by virtue of a
steer input by the auxiliary steering system, i.e. a more rapid brake
pressure buildup is permitted at the wheel with the higher friction
coefficient, but there is no modification of the absolute value of the
brake pressure difference between the wheel on the side with the higher
friction coefficient and the wheel on the side with the lower friction
coefficient.

[0016] Overall, therefore, higher braking forces can be produced by the
exemplary embodiments and/or exemplary methods of the present invention
in the case of braking operations under split-coefficient conditions if
the driver and/or an auxiliary steering system make stabilizing
interventions.

[0017] Here, the stabilizing steer input is performed by the driver or
automatically by an auxiliary steering system. The auxiliary steering
system can assist the driver by specifying a steering recommendation,
e.g. by a steering torque that can be felt by the driver at the steering
wheel. The control unit of the auxiliary steering system obtains
information on the presence of a braking yaw moment in the case of a
braking operation under split-coefficient conditions from a yaw rate
sensor, for example. A steer input by the driver can be detected by a
steering angle sensor, for example. Both variables--steering angle and
yaw rate--therefore allow an inference as to the currently available
level of stability of the vehicle (yaw rate) and as to the input that has
been made by the driver or the auxiliary steering system (steering angle)
in the case of a braking operation under split-coefficient conditions. In
this situation, the two variables influence each other. Thus, a "correct"
steer input as a response to the braking yaw moment can leads to a
reduction in the actual yaw rate of the vehicle. Otherwise, a high actual
yaw rate as a consequence of a high braking yaw moment can require a
larger steer input. Moreover, it is, of course, possible for "incorrect"
steer inputs as a response to the braking yaw moment to lead to an
increase in the actual yaw rate.

[0018] Advantageous developments and improvements of the exemplary
embodiments and/or exemplary methods of the present invention indicated
in the description herein are possible by the measures presented in the
further descriptions herein.

[0019] In a particular embodiment, the actual yaw rate of the vehicle is
adjusted by closed-loop control to a setpoint yaw rate as a function of
the steer input by the driver and/or by the auxiliary steering system by
modifying the brake pressure difference or the differential brake
pressure value. In this case, the yaw rate represents a controlled
variable, the steer input represents a disturbance and the brake pressure
difference represents a manipulated variable of the closed-loop control
system. By this closed-loop control, the stability of the vehicle is
given priority, and the brake pressure difference at the axle is adapted
on an ongoing or continuous basis so as to minimize the system deviation.

[0020] For carrying out the method, the exemplary embodiments and/or
exemplary methods of the present invention provides a device which
comprises at least one brake actuator per wheel of the axle, said brake
actuator being controllable electrically, directly or indirectly, and
being actuated by pressure medium, at least one brake pressure sensor per
brake actuator for the purpose of producing sensor signals dependent on
the brake pressure acting in the respective brake actuator, at least one
steering angle sensor for producing sensor signals dependent on the steer
input, at least one yaw rate sensor for producing sensor signals
dependent on the yaw rate of the vehicle or on yaw rates acting on the
vehicle, and at least one electronic control unit, which adapts the brake
pressure difference or differential brake pressure value between the
brake pressure at the wheel with the higher friction coefficient and the
brake pressure at the wheel with the lower friction coefficient as a
function of the sensor signals produced by the steering angle sensor, the
yaw rate sensor and the brake pressure sensors. These components are
present in any case as part of vehicle dynamics control systems (ESP),
and therefore no additional outlay on construction is required for the
device.

[0021] An auxiliary steering system controlled by the control unit in such
a way that a yaw moment acting counter to the braking yaw moment is
produced at the steering wheel, eliminating the difference between the
actual yaw rate and the setpoint yaw rate, may also be provided.

[0022] Further measures that improve the exemplary embodiments and/or
exemplary methods of the present invention are explained in greater
detail below with reference to the drawing, together with the description
of an illustrative embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The Figure shows a block diagram which illustrates the method in
accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

[0024] The block diagram in the figure illustrates an exemplary embodiment
of a method for operating a brake device, e.g. an electropneumatic brake
device, or an electronic brake system (EBS) of a commercial vehicle with
an antilock brake system (ABS) on roadways with different friction
coefficients on different sides. Owing to the difference in the friction
coefficients on different sides, a braking yaw moment on the vehicle is
produced during braking, resulting in an actual yaw rate, "yaw rate",
that can be measured by a yaw rate sensor.

[0025] On, for example, a front axle of the commercial vehicle, an
absolute brake pressure difference or absolute differential brake
pressure value between the brake pressure value at the wheel on the end
of the axle with the higher friction coefficient and the brake pressure
value at the wheel on the end of the axle with the lower friction
coefficient is adapted as a function of a steer input, intended to
produce a yaw moment acting counter to the braking yaw moment, by the
driver and by an automatically intervening auxiliary steering system.

[0026] As a particularl option, the actual yaw rate, "yaw rate", of the
vehicle is adjusted by closed-loop control to a setpoint yaw rate as a
function of the steer input by the driver and by the auxiliary steering
system by modifying the differential brake pressure value to a setpoint
yaw rate. In this case, the yaw rate represents the controlled variable,
the steer input represents the disturbance and the brake pressure
difference represents a manipulated variable of the control system. The
block labeled "vehicle geometry" in the figure symbolizes the controlled
system.

[0027] To achieve control, there is one brake actuator per wheel of the
front axle, said brake actuator being controllable electrically, directly
or indirectly, and being actuated by pressure medium, one brake pressure
sensor per brake actuator for the purpose of producing sensor signals
dependent on the brake pressure acting in the respective brake actuator,
a steering angle sensor for producing sensor signals dependent on the
steer input, a yaw rate sensor for producing sensor signals dependent on
the yaw rate of the vehicle or on yaw rates acting on the vehicle, and an
ABS control unit, which is denoted by "ABS" in the figure and forms the
controller.

[0028] The ABS control unit, "ABS", adapts the brake pressure difference,
"brake pressure", between the brake pressure at the wheel with the higher
friction coefficient and the brake pressure at the wheel with the lower
friction coefficient as a function of the sensor signals produced by the
steering angle sensor, the yaw rate sensor and the brake pressure
sensors.

[0029] There may be also an auxiliary steering system which is controlled
by the ABS control unit, "ABS", and, for compensation, produces a yaw
moment at the steering wheel acting counter to the braking yaw moment,
which is also denoted in the figure as "steering recommendation". This
"steering recommendation" is intended to eliminate the difference between
the actual yaw rate ("yaw rate") and the setpoint yaw rate. In addition,
the driver can furthermore also perform stabilizing steer inputs, this
being denoted by "driver response" in the figure.

[0030] During a braking operation under split-coefficient conditions, a
braking yaw moment consequently arises, this being detected by the yaw
rate sensor and being reported to the ABS control unit, "ABS", by a
corresponding signal, "yaw rate". The control unit compares the actual
yaw rate, "yaw rate", with a setpoint yaw rate, which is equal to zero in
straight ahead travel, for example. At the same time, the ABS control
unit, "ABS", controls the auxiliary steering system in order to produce
an opposing yaw moment to compensate for the braking yaw moment. This yaw
moment can also be felt by the driver at the steering wheel, and the
driver can either leave the yaw moment specified by the auxiliary
steering system unchanged without intervening or can modify it by
intervening ("driver response"). The block labeled "vehicle" or "vehicle
geometry" then receives a "steering angle" specified by the steer input
by the driver and/or by the auxiliary steering system ("steering
recommendation") and a "brake pressure" (brake pressure difference) as
input variables, which lead to a particular actual yaw rate, "yaw rate",
along the controlled system, "vehicle geometry".

[0031] Thus, a "correct" steer input as a response to the braking yaw
moment can leads to a reduction in the actual yaw rate, "yaw rate" of the
vehicle. As the yaw rate decreases, the permissible brake pressure
difference, "brake pressure", is simultaneously increased in order to
enable more braking power to be transmitted, with the associated increase
in driving stability.

[0032] It is clear here, however, that a high actual yaw rate as a
consequence of a high braking yaw moment requires a larger compensating
steer input or steering angle.

[0033] Otherwise, that is to say when there is an "incorrect" steer input
in the presence of a braking yaw moment, e.g. when the driver overrides
the "steering recommendation" of the auxiliary steering system by a
manual steer input, the actual yaw rate, "yaw rate", will increase
undesirably, thus further reducing driving stability. To the extent that
instability is increased by an increase in the actual yaw rate, "yaw
rate", however, the permissible differential brake pressure value, "brake
pressure", is reduced in order to avoid further increasing the
instability or to reduce the braking yaw moment through a reduced
asymmetry of the braking forces.

[0034] According to another embodiment, there is no auxiliary steering
system. In this case, it is the driver's responsibility alone to
compensate for the braking yaw moment through his steer inputs.